1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to a liquid crystal display device in which an orientation control function is provided to an orientation film by light irradiation.
2. Description of the Related Art
As a method of performing an orientation treatment, that is, providing an orientation control function to an orientation film used in a liquid crystal display device, there has been a method of performing a rubbing treatment. In the orientation treatment by rubbing, the orientation film is rubbed by a cloth in a specified direction so that the orientation treatment is performed. On the other hand, as a method of providing the orientation control function to the orientation film in a non-contact manner, there is a light orientation method.
Patent document 1 discloses a technique in which an orientation film is irradiated with ultraviolet polarized light, and an excited state by a light energy reaction is used to cause a new bond formation reaction or bond cleavage reaction to occur in molecular chains of the orientation film, so that a pretilt angle and directionality are imparted to liquid crystal molecules.
[Patent document 1] JP-A-7-318942 (relevant patent: U.S. Pat. No. 5,604,615)
This kind of orientation film is formed of a coating film of polyimide resin. In the orientation film which uses polyimide resin with alkyl side chains and to which the orientation control function is provided by light orientation, the pretilt angle can be generated by inclining the irradiation direction of the polarized light relative to the substrate surface. However, in the related art, the generation amount of fine decomposition product by cutting of ultraviolet rays is not considered. The decomposition product reduces afterimage characteristics, thermal stability or electric stability and has been a factor to degrade the display performance according to the amount thereof.
Specifically, when the orientation film has an ion adsorption site, an ion in the liquid crystal repeats the adsorption to and release from the adsorption site, so that the afterimage characteristics due to a direct current component (DC) is degraded in the orientation film. When a large amount of decomposition product (short molecule component) remains in the orientation film, following phenomena are caused.
(1) Since the short molecule component has a low molecular weight, a part of the residue exceeding a specific amount dissolves in the liquid crystal, and the voltage holding ratio of the liquid crystal molecule orientation is reduced.
(2) When it is assumed that the short molecule component is something like shavings by rubbing, a large amount of the remaining component causes reduction of the elastic modulus of the surface of the orientation film.
(3) Since a large amount of the remaining short molecule component does not contribute to the orientation control, the thermal stability and electric characteristics of the orientation film are reduced.